A typical active impedance line feed circuit includes tip and ring amplifier circuits being controlled to exhibit predetermined a.c. impedance and d.c. resistance characteristics for the purpose of supplying energizing current for a two wire communication line. These characteristics are determined by respective a.c. and d.c. feedback networks in association with an amplifier in the line feed circuit. One problem associated especially with an integrated circuit realization of an active impedance line circuit is that of heat dissipation. This problem can be extreme, particularly in an equipment shelf or housing containing a densely packaged group or groups of the active impedance line feed circuits. In one example of the in line feed circuit in application Ser. No. 386,871 filed 10 June, 1982 a diode is combined with the d.c. feedback network to limit feed current in the case of a short and hence low resistance communication line. As the line feed curent is thus limited in some of the line feed circuits in a typical group of the line feed circuits, the overall heat dissipation problem is likewise advantageously reduced. However, an occasional occurrence of a ground fault on the communication line may well cause individual heat induced circuit failure and in an extreme case failure of immediately adjacent circuits.
The potential severity of this problem is illustrated in the case of a low resistive fault connection from the ring terminal of an active impedance line circuit to ground, the communication line being otherwise open circuited, i.e., ON HOOK. In this case, current flows only in the ring feed resistor and the ring amplifier circuit of the line feed circuit. In this case, current limiting in the ring amplifier will occur only at about twice the level of normal loop current. Furthermore resultant heat generation is disadvantageously concentrated in one small area, that is at the ring amplifier circuit portion of the line feed circuit.